Oooh that makes sense. From what I understand, with Steam each environment is installed to its own ManiaPlanet game folder, instead of being all together.
TMPlus combines all environments (Canyon, Valley, Stadium) into one title. In order to use it, you need to have all three environments installed and running in your current ManiaPlanet instance.
So it seems TMPlus (and probably any other multi-environment titlepack) is incompatible with Steam.